Circuit for measuring the last half cycle voltage across a resistance weld



May 4, 1948.

O. W. LIVINGSTON CIRCUIT FOR MEASURING THE LAST HALF CYCLE VOLTAGEACROSS A RESISTANCE WELD Filed lay 4. 1944 2 Sheets-Sheet 1 TIMERInvehtor: Orrin W. Livin ston,

Hus Attofney .May 4, 1948.

o. w. LIVINGSTON 2,440,962

CIRCUIT FOR MEASURING THE LAST HALF CYCLE VOLTAGE ACROSS A RESISTANCEWELD Filed lay 4, 1944 2 Sheets-Sheet 2' Inventor: OrPinW. Livingston yHis Attorney.

UNITED STATES PATENT OFFICE omonrr For. MEASURING THE LAST HALF crcmVOLTAGE Acaoss A RESISTANCE Orrin W. Livingston, Scotia, N. Y., assignorto General Electric Company, a corporation of New York Application May4, 1944, Serial No. 534,120

20 Claims. (01. 171-95) My invention relates to electric controlcircuits and more particularly to improved control circuits adapted forobtaining a measurement of the last half cycle value of an electricalcondition provide a new and improved electric control or indicatingcircuit.

It is a further object of my invention to provide a new and improvedelectric control circuit for indicating the last half cycle value of anelectrical condition that occurs during an enerthat occurs in a loadcircuit during a period of gization period of an intermittentlyenergized alenergization thereof and for maintaining said ternatingcurrent load circuit and for maintaining measurement during theimmediately following said indication during the immediately followingperiod 01' deenergization ofsaid load circuit. period of deenergizationof said load circuit.

Although of general utility, circuits embodying It is also an object ofmy invention to provide my invention are particularly suited forindicata new and improved indicating circuit including ing the qualityof resistance spot welds formed a capacitor which is charged in responseto an between parts by applying thereto a predeterelectrical conditionthat occurs in an alternating mined time application of welding currentflow current load circuit during half cycles of a polarity through theagency 01' pressure exerting eleccorresponding to that of the last halfcycle of an trodes. energization period and which is discharged in Aspointed out in application Serial No. 534,010, response to half cyclesof opposite polarity. Eugene R. Spittler and Ivar W.-Johnson for Re- Itis also an object of my invention to provide sistance welding andresistance weld testing filed an electric control circuit which may beused for concurrently herewith and assigned to the same indicating thequality of a resistance spot weld assignee as this invention, it hasbeen discovered immediately after its formation. that the variation inclosed circuit voltage across Further objects of my invention willbecome a resistance weld during its formation is a satisapparent fromthe following description of two factory criterion of its quality. Asmore fully embodiments thereof diagrammatically repredescribed in thatapplication, it has been detersented in Figs. 1 and 2 01 theaccompanying mined that in the case of ferrous metals the as drawings.voltage across the weld increases in value as the Each control circuitillustrated in these drawweld is formed and becomes substantiallyconings is in reality a special type of vacuum tube stant upon theformation of a satisfactory weld. voltmeter whose function is that ofindicating Likewise, in the o at o of m ne rous e ds, the magnitude ofthe voltage across a resistance Such as Welds between Parts 05 a u um,weld for the last half cycle of alternating curaluminum alloys, brassand the like, it has also rent flow and also that of providing an alarmbeen determined that the voltage across the weld for warning theoperator of the formation of an decreases in value as the weld is formedand unsatisfactory weld. A capacitor forming a part again attains asubstantially constant value when of each system is charged during awelding opa weld of satisfactory quality has been formed. eration inaccordance with the maximum voltage Methods of controlling resistancewelding operavalue occurring during half cycles of a polarity tions inaccordance with the substantially concorresponding to that of the lasthalf cycle of stant value of voltage at the end of a weld as anenergization period of the welding circuit and well as the voltageacross the weld at the end of is discharged during half cycles of apolarity opthe welding operation have been claimed in this posite t t oft m t h m cycle of t energiza- Spittler 8; Johnson application. Sincethe tion period of the welding circuit. At the end magnitude of thevoltage across a resistance weld of a welding period the last half cyclecharge on for the last half cycle of welding time may be the capacitoris maintained during the immeused as a measure of weld quality, circuitsemdiately following period of deenergization of the bodying my inventionmay be used for measuring welding circuit and means are provided forindithis last half cycle value of voltage and compareating the value ofthis charge. Means are also ing it with a predetermined value for likewelds provided for comparing the magnitude of this of desired quality inorder to determine the quality capacitor charge with a predeterminedstandard of the weld being formed. Such circuits prefvalue for welds ofdesired quality and for warning erably include, in addition to anindicating de- 59 the welding operator when the last half cycle vice, asuitable alarm system which will inform value of voltage for a weldingperiod does not the welding operator when an unsatisfactory weld fallwithin desired operational limits and consehas been made. quentlyfurnishes evidence of the formation of an It is consequently an objectof my invention to unsatisfactory weld.

In the system shown in Fig. 1 the electrodes 1 of a resistance weldingmachine are connected 3 to the terminals of the transformer 3 having aprimary winding 4. The primary winding 4 of the welding transformer isconnected to a source of alternating current sup-- ply 5 throughconductors 6 and a pair of electric instant in each cycle that theassociated valve is rendered conductive. The excitation circuits forcontrolling the energization of these control elesecondary 2 of awelding ments may be of any of the types well known in trol elements 9which is in phase opposition to the anode-cathode voltages of electricvalves i and 8 and transformers ii and H for supplying components ofalternating current voltage to these control elements which is in phasewith their anode-cathode voltages.

Transformer it forms part of a timer l3 by means of which electricvalves 1 and 8 are intermittently energized from the alternating currentsupply circuit 5 when energization is initiated by closure of a manuallyoperated switch 84. Transformer i2 forms partofa heat control i5 bymeans of which a component of voltage derived from transformer i2 isadjusted in phase with respect tothe anode-cathode voltages of electricvalves '5 and 8 in order to control the magnitude of the current whichit is desired to supply through these valves to the load oircult. Therelative magnitudes of the control voltages in the excitation circuitsof electric valves i and 3 is such that both the iii-phase componentssupplied by transformers and l2 7 are required to render electric valves77 and 3 conducting. In this way the timer determines the cycles duringwhich the valves conduct and the phase of the voltage derived from heatcontrol I5 determines the time in the cycle that con duction isinitiated. The component of voltage supplied by transformer 62 ispreferably of peaked wave form. Heat control it may also embody currentregulating means and may be any suitable type, for example thatdescribed and claimed in the copending application of Benjamin Cooper,Serial No'. 480,150, filed March 23, 1943, assigned to the assignee ofthis invention, and now Patent No. 2,421,994, granted June 10, 1947.

As will be understood by those skilled in the art, the resistancewelding system thus far described is placed in operation by closingmanual switch I. Thereafter, welding transformer 3 is energized for aninterval or a succession of intervals under the control of timer i3 witha. cur- I a 4 points near their work enga ing surfaces. As illustrated.an adjustable connection may be provided in order to vary the number ofprimary turns of this transformer which are connected across the weldingelectrodes. The terminals of the secondary I! of transformer i8 areconnected tothe parallel charging circuits for capacitors l9 and 20.

Capacitor I9 is charged through a unidirectional current conductingdevice 2! which is poled to pass current to the capacitorduring halfcycles of a polarity corresponding to the last half cycle of anenergization period of the load circult which in the arrangementillustrated comprises the weld being formed between electrodes 8 of theresistance welding machine. Capacitor it is charged in the same mannerthrough a resistor22 and a second unidirectional current conductingdevice 23 which is poled in the same manner as unidirectional currentconducting device 2i. Devices-2i and 23 may be of any suitable type andhave been illustrated in the drawing as electric discharge devices.

Capacitor i9 is employed for measuring the voltage across the weld andcapacitor 2d forms part of a time delay circuit including a relay 24having contacts 25 in the charging circuit of capacitor it. This relayoperates in a manner to close the charging circuit of capacitor is whenwelding current flows and to open this charging circuit a short intervalof time after the fiow of welding current ceases and before electrodes iof the welding machine are withdrawn from the work. Consequently, anysparking between the welding electrodes and the work at the time theelectrodes disengage the work is prevented from increasing the charge oncapacitor is which, as will be pointed out below, has a value dependingupon the maximum value of the last half cycle of voltage across theelectrodes at the end of a weld period.

'Iihe operating winding 25 of relay 213 is connected in theanode-cathode circuit of a threeeiernent electric valve 28 of the highvacuum type and in circuit with a resistance element 2? across aresistance element 29. This resistance element forms part of a voltagedivider comprising resistance elements 29 and 3d connected in serieswith one another across a source of direct current voltage iii, 32.Control element 33 of electric valve 28 is connected through aresistance elemeat 33' to the positive terminal of capacitor it. Thiscapacitor is also provided with a discharge circuit including aresistance element 36.

The source of direct current voltage all, 32 may be obtained in anysuitable manner. In the arrangement illustrated, it is obtained from avoltage divider including a resistance element 35, a potentiometer S6,and a resistance element 371 connected in series with one another acrossthe outputcircuit of a full-wave rectifier. This rectifier is connectedto the source of alternating current supply 5 by means of a transformer38 having a mid-tapped secondary winding 39 conpositive half cycles andproduce a charge on these capacitors. assuming of course that thecircuit for capacitor 18 is completed through the contacts 25 of relay24. During the first positive half cycle, the charge on capacitor 24will become sumcient to overcome the negative bias voltage appearingacross resistance element 30 between control element 33 and the cathodeof electric valve 28 and thereby render this valve conducting toenergize the operating winding 28 of relay 24. This relay will thereuponclose its contacts 25 and complete the charging circuit of capacitor I9.At the end of a welding period, capacitor 20 will be discharged throughits parallel-connected resistor 34 and the negative bias acrossresistance element 35 will again render electric valve 28 non-conductingthereby causing relay 24 to open its contacts 25. Thereafter when theelectrodes I are separated from the work, if any sparking occurs betweenthem and the work due to the inductance of the secondary circuit of thewelding transformer, the voltage across these electrodes during suchsparking can in no way increase the charge imparted to capacitor [8 bythe last half cycle of welding current flow.

Capacitor I5 is discharged in response to half cycles of a polarityopposite that of the last half cycle of an energi-zation period of theresistance welding transformer 3 through the agency of an electric valve45 of the type employing an ionizable medium such 'as a gas or vapor.The anode-cathode circuit of electric valve 45 is connected in circuitwith a current limiting resistance element 48 across the terminals ofcapacitor iii. In order to insure that electric valve 45 commutates outsuccessfully after discharging capacitor 5, a series connectedresistance element 4'! and capacitor 48 are connected in theanode-cathode circuit of this valve across load limiting resistanceelement 45.

The control element 49 of electric valve 45 is connected throughresistance elements 50, 5E and 3! to its cathode. Resistance element 31is shunted by a capacitor 52 and resistance element 5! is shunted by thesecondary winding 53 of a control transformer 54. Capacitor 52 serves tostifien the bias voltage across resistance element 31' and resistanceelement 5! serves to dampen oscillations in the circuit of secondarywinding 53 of control transformer 54.

The primary of control transformer 54 is ac tuallythe secondary of thewelding transformer 3. In the arrangement illustrated this result hasbeen obtained by placing its magnetic core structure 55 about one of theelectrodes l and supporting it on the arm of the welding machine bymeans of which this electrode is supported. This transformer 54 is a.saturable inductive device, that is its core structure 55 is readilysaturated by small values of current flow in the welding circuit.Consequently, the output voltage of its secondary 53, which also appearsacross re sistance element 5|, is a sharp peak affair which occurs whenthe current in the welding circuit passes through zero value. When thewelding current passes through zero, a single peak will be produced iffull half cycles of current are flowing. If, due to heat control, lessthan full half cycles of current are flowing, there will be two peaks oflike po'larity occurring when the current decreases to zero value andsubsequently after a period of no current flow, when the currentincreases from zero value. At such times the peak voltage wave derivedfrom control transformer 54, when of proper polarity, overcomes thenegative bias voltage across resistance element 3'! and capacitor 52 torender electric valve 45 conductive and thereby discharge capacitor IS.The secondary winding 53 of control transformer 54 is connected so thatat the end of the last half cycle of current flow in the weldingcircuit, a positive peak of voltage will not be produced andconsequently will not render electric valve 45 conductive so that thefinal charge on capacitor l9 due to the last half cycle of weldingcurrent flow will be maintained during the immediately following periodof, deenergization of the welding circuit.

The voltage value of capacitor I9 is indicated by a milliammeter 55connected in circuit with a resistance element 58, a potentiometer 51,and the anode-cathode circuit of an electric valve 58 across the directcurrent source of supply 3!, 52. The control element 59 of electricvalve 58 is connected through a resistance element 50 to the positiveterminal of capacitor I9. Electric valve 58 is of the high vacuum typeand is connected in a cathode following manner to record the voltageacross capacitor l9. In order to provide a range adjustment formilliammeter 55, which is functioning as a voltmeter, resistance element56 may be made adjustable.

Means may be provided for operating a signal when the voltage ofcapacitor l9 due to the last half cycle of welding current flow is notwithin certain operational limits previously established as necessaryfor obtaining a satisfactory weld. In the system of Fig. 1 the signaldevice is an electric discharge device 6i which is illuminated only whenthe voltage of condenser is is within the limits established for a goodweld. This electric valve Si is connected across a resistance element 52in circuit with a resistance element 53, and the anode-cathode circuitof an electric valve 54 across the secondary 55 of transformer Theanode-cathode circuit of an electric valve 66 is connected in shunt tothe anode=cathode circuits of the series connected electric valves 6iand 64. Consequently, when electric valve 64 becomes conductive,electric valve M will be illuminated by reason of its connection acrossthe secondary 65 of transformer 38. On the other hand, whenelectric'valve 65 becomes conductive, electric valve Eil is shunted downand thereby extinguished. These electric valves iii, 64, and B6 arepreferably of the type employing anionizing medium such as a gas orvapor.

The cathodes of electric valves 54 and 68 are connected to the slider ofpotentiometer 36 forming part of the voltage divider 35, 36, 37. Thecontrol element 61 of electric valve 54 is connected through resistanceelement 88 to a terminal 69 of the cathode following circuit embodyingelectric valve 58 and the control element 10 of electric valve 66 isconnected through a resistance element 10 to the slider of potentiometer5! in this cathode following circuit. Consequently, the position of theslider on pctentiometer 35 determines the lower limit of voltage in thecathode following circuit at terminal'69 which will render electricvalve 54 conductive and the position of the slider on potentiometer 51determines the upper limit of voltage in the cathode following circuitwhich will not render electric valve 55 conductive. Thus, if the voltageof condenser I9 and consequently the voltage of terminal 69 in thecathode following circuit does not equal the value of voltage determinedby the position of the slider on potenposition of the slider onpotentiometer 51 in the cathode following circuit will be sumcient torender electric valve conductive, and byshunting electric valves BI and64 will extinguish electric valve BI and again indicate the formation ofan unsatisfactory weld. Only when the voltage of the capacitor [9 andthe terminal 69 of the cathode following circuit of electric valve 58 iswithin'the limits established by potentiometers 36 and will the electricvalve it be illuminated, indicating the formation of a satisfactoryweld.

In the system illustrated electric valves 45, 64 and 66 are providedwith shielding grids which are connected to their cathodes. Transientsuppressing capacitors are also connected between the control elementsand cathodes of these electric valves.

Although the operation of the system of Fig. 1 is believed to beapparent from theforegoing detailed description thereof, it may bebriefly summarized as follows: K

Closure of initiating switch it will cause welding transformer 3 to beenergized one or more times from the source of alternating currentsupply 5 depending on the control exerted on electric valves l and B bytimer it. Each period of energization will last for a predeterminednumber of half cycles ending with a half cycle of the same polarity andthe periods of energization will be separated from one another bypredetermined periods of deenergization if more than one period ofenergization is occasioned by timer H3. The magnitude of the currentflow will depend upon the adjustment of heat control i5.

' Assuming that the last half cycle value of an energization period ofthe welding circuit is of positive polarity, control transformers it and5d of the indicating circuit are properly connected to charge capacitorsI9 and 2!] during positive half cycles of current flow and to dischargecapacitor it at the end of a negative half cycle of current flow in thewelding circuit.

The first positive half cycle of current how in the welding circuit willinduce a voltage in sec ondary l8'of control transformer H5 in a direc--tion to charge capacitor 2t through unidirectional current conductingdevice 23 and resistance element 22. The charge thus imparted tocapacitor 20 will overcome the negative bias voltage across resistanceelement 30 and impress between control element 33- and the cathode ofelectric valve 28 a positive value of voltage sufficient to render thisvalve conducting. Conductivity of electric valve 28 will energizeoperating winding 26 of relay 24 through resistance element 21 inresponse to the direct current voltage appearing across resistanceelement 29 of voltage divider 29, 30 connected across the direct currentsource of supply 3|, 32. This. energization of relay 26 will cause it toclose its con= tacts 25, thereby completing the charging circuit forcapacitor l9. Consequently, the next positive half cycle of current flowin the welding circuit will charge capacitor ls through theunidirectional current conducting device 2| to a value of voltagecorresponding to that of the maximum voltage occurring across weldingelectrodes I. a

At the end of the first negative half cycle of current flow in thewelding circuit, transformer 6Q will produce a positive peak of voltagein its secondary winding 53. This secondary winding 53 is connectedacross-a resistance element 5| in the contro1 circuit of electric valve45 and the positive peak of control voltage generated in its secondarywinding 53 and appearing across resistance element an will be sufllcientto overcome the negative bias voltage appearing across resistanceelement all and capacitor 52 also connected in the excitation circuit ofelectric valve 45. Consequently, control element 48 of electric valve 45will become positive relative to its cathode causing it to becomeconductive and discharge capacitor it through resistance element 46. If,due to heat control less than full half cycles of current how in thewelding circuit, there will be a second positive peak of voltage inducedin secondary 53 of transformer 54 at the beginning of the next positivehalf cycle of current flow but this will have no useful effect .sincethe first positive voltage peak will have caused electric valve 35 todischarge capacitor i 9. Resistance element l'l'connected in series withcapacitor 58 across resistance element at will cause electric valve 135to commutate successfully after discharge of capacitor l9.

Thereafter, during positive half cycles of current flow in the weldingcircuit capacitor it will be charged and at the end of negative halfcycles of current flow it will be discharged. Consequently, at the endof a welding period which has been assumed ends with a half cycle ofpositive polarity, capacitor is will be left with a charge depending onthe maximum value of the voltage across welding electrodes l.

' Shortly after the welding period ends, capacitor 29 will havedischarged suficiently through resistance element 33 to reduce thecurrent flow through electric valve 28 to a value that will cause relay2&3 to open its contacts 25. -Consequently, any residual voltage of thewelding circuit will not be impressed on capacitor 99 when electrodes iare separated from the work and thereby give a false indication of themagnitude of the last half cycle value of welding current flowpreviously recorded as a chargev on the capacitor. To accomplish thispurpose, it is only necessary that before the welding electrodes areseparated from the work after a welding operation, capacitor 2t bedischarged sufficiently through the circuit including resistance element36 to cause electric valve 2a to decrease the energization of relay 2%enough to cause it to open lat; contacts 25 in the charging circuit ofcapacitor Control element 59 of electric valve 58 is connected to beresponsive to the charge on capacitor 89. Consequently, at the end of awelding period electric valve 58 will .be rendered conductive inaccordance with the charge on capacitor l9 causing a current flowthrough milliammeter 55 corresponding in magnitude to the voltage ofcapacitor IS. The deflection of the pointer of milliammeter 55 will thusindicate the magnitude of the voltage'of capacitor i9. This indicationwill be maintained during the deenergi'zatlon period of the weldingcircuit and consequently may be readily observed by the welding operatorafter each welding operation.

minai 69 of the cathode following circuit including electric valve 88 toattain a corresponding value. This voltage value is applied to controlelement 81 of electric valve 64 and if greater than the cathodepotential of this valve will cause it to become conductive, therebyconnecting electric valve 8| across the secondary 65 of transformer 38.The minimum value of voltage necessary to render electric valve 64conductin is determined by the cathode potential thereof and this inturn is determined by the position of the slider of potentiometer 88 ofthe voltage divider 35, 36, and 31. Consequently, if the voltage ofterminal 69 does not attain the value determined by the position of theslider on potentiometer 36, electric valve 64 will not become conductiveand electric valve 8| will not be illuminated by being connected acrosssecondary 65 of transformer 38. Potentiometer 38 thus furnishes anadjustment for the minimum value of voltage across capacitor I9 whichwill operate signal device 6|.

The cathode potential of electric valve 65 is also determined by theadjustment of the slider on potentiometer 36. The voltage applied to itscontrol element 10 however is dependent upon the adjustment of theslider of potentiometer 51 in the cathode following circuit includingelectric valve 58. Consequently, if the potential of terminal 59 of thiscathode following circuit increases beyond a predetermined value for agiven adjustment of potentiometer 51, electric valve 66 will becomeconductive. When electric valve 86 becomes conductive, it completes ashunt circuit about electric valves BI and 64 and thus extinguisheselectric valve 6i whose circuit has been previously completed byelectric valve 64 becoming conductive. Consequently, if the voltageacross capacitor I9 is greater than a predetermined value determined bythe position of the slider of potentiometer 51, signal device BI is notenergized. It is thus seen that only when the voltage of capacitor I9 isbetween predetermined limits is signal device 6| operated to inform thewelder that a satisfactory weld has been made.

When the welding circuit is again energized to makethe next weld, thefirst half cycle of negative current flow will, in the manner described,cause electric valve 45 to discharge capacitor l9. Thereafter theoperation of the circuit will be as above described and at the end ofthe next welding operation, capacitor l9 will acquire a chargeindicative of the quality of the next weld thus formed.

In Fig. 2 I have illustrated another embodiment of my invention. Theload circuit including the welding transformer is-the same as thatillustrated in Fig. 1 of the drawings, and like reference numbers havebeen applied to corresponding parts. Fig.2 of the drawings has beensimplified by omitting therefrom the excitation control circuits forelectric valves 1 and 8 connected in the load circuit. It is to beunderstood that an arrangement similar to that-shown in Fig. 1 or someother suitable arrangement will be employed to control the intermittentenergizations of the welding transformer 3 from the source ofalternating current supply 5.

The capacitor ll of Fig. 2 corresponds in function to capacitor IQ ofFig. 1, above described. It is charged during half cycles of currentflow corresponding in polarity to the last half-cycle of a weldingperiod and discharged during half cycles of current flow of the oppositepolarity.

Its charging circuit is connected to be responsive to the voltage acrossthe welding electrodes l. Thus, at the end of a welding period it hasacquired a charge corresponding in magnitude to the maximum value of thelast half cycle of voltage across the welding electrodes.

In Fig. 2 the time element relay 24 of Fig. 1 has also been omitted inorder to simplify the drawing. It is to be understood however that it oran equivalent device may be used when the welding operation is soconducted that residual voltage in the welding circuit gives a falseindication of the magnitude of the last half cycle of welding currentflow previously recorded as a charge on capacitor H.

As in Fig. 1 the charge on capacitor H is applied through a resistanceelement '12 to the control element 13 of an electric valve 14, formingpart of an indicating circuit. This indicating circuit includes amilliammeter 15, a resistance element 18, and electric valve 14connected in series with one another across a source of direct currentsupply I1, 18. The conductivity of electric valve 14, which is of' thehigh vacuum type, will be controlled by the charge on the capacitor Hand the resulting flow of current in the indicating circuit will causethe pointer of the milliammeter to be deflected in accordance with thevoltage of capacitor l I. Thus the milliammeter 15 functions as avoltmeter and during the period of deenergization of the welding circuitwill indicate the value of the last half cycle of voltage across I thewelding electrodes during the immediately preceding welding period.

The direct current voltage ll, '18 for the indicating circuit may beobtained in any suitable manner. In Fig. 2 it has been obtained from avoltage divider circuit including resistance elements 19, 80, and 85.These resistance elements are connected in series with one anotheracross the output terminals of a full wave rectifier in cluding anelectric discharge device 82 and a. transformer 83. The output of thisrectifier'is suitably filtered by inductances 84 and capacitors 85.

Capacitor H is connected through a unidirectional current conductingdevice 88 to one of the secondary windings 81 of a control transformer88 whose primary 89 is connected across the welding electrodes I.Control transformer 88 and'unidirectional conducting device 88 areconnected so that capacitor H is charged only in response to half cyclesof voltage across welding electrodes I corresponding in polarity to thelast half cycle of welding current flow in the load or welding circuit.

In order to avoid a direct current saturation effect in transformer 88,its secondary winding 8! is also connected to a loading circuitincluding a unidirectional current conducting device 90 and anadjustable resistance element 9i. Unidirectional current conductingdevice 90 is poled to pass half cycles of current having a. polarityopposite to that for which unidirectional current conducting device 88is poled. Thus by reason of the loading circuit, current is allowed toflow in both directions through secondary winding Bl of controltransformer 88 and the amount of current flowing in the loading circuitmay be adjusted to a value equivalent to that flowing in the chargingcircuit of capacitor H. Unidirectional current conducting devices 86 and90 may be of any suitable type, and have been illustrated as electricdischarge devices. Primary 89 of control transformer 88 has beenillustrated as being provided with an adiustable connection 82 in orderto vary the number of p turns connected'across the welding electrodes,

Capacitor II is discharged by means of an electric valve 88 in responseto half cycles of current flow in the welding circuit which are of apolarity opposite that of the last half-cycle control transformer 88having a primary winding 88. These control voltages are introduced intotwo parallel connected branches of an'excitation circuit for electricvalve 88. One branch of this excitation circuit is polarized by aunidirectional current conducting device I88. This device has beenillustrated as an electric discharge device having its anode directlyconnected to the control element 88 of electric valve 83 and its cathodeconnected toward the cathode of electric valve 83 in circuit with thesecondary winding- 81 of control transformer 88 and the negative biasvoltage across resistance elements 8| and 88 of voltage dividers I8, 88,and 8|. The positive terminal of the bias voltage across resistanceelement 88 is connected to the cathode of electric valve 88 and itsnegative terminal is connected through secondary 88 of controltransformer 88, and a resistance element IM to the control element 88 ofelectric valve 88. It is to be noted that electric discharge device I88is poled to conduct current from the nonpolarized excitation circuit ofelectric valve 88 through the resistance element I8I thereof.

The primary 88 of control transformer 88 is connected to a phase shiftnetwork I82 through a variable resistance element I88 and a seriesresonant circuit including a non-saturable inductive element I84 and acapacitor I88. This resonant circuit is tuned approximately to thefrequency of, the supply circuit 8 by means of which the phase shiftnetwork I82 is energized. This phase shift network includes anadjustable resistance element I88 connected in series with a capacitorI81 across the primary winding I88 of transformer83. .The series circuitincluding primary winding 88 of transformer 88, adjustable resistanceI88, inductance I88, and capacitor I88 is connected by means ofconductors I88 across the common connection of capacitor I81 withresistance element I88 and the midpoint of the primary winding I88 oftransformer 83. Control transformer 88 is a saturable inductive devicehaving an output voltage of peaked wave form. The control transformer 88and its energizing connections forms the subject matter of United StatesLetters Patent 2,246,177, Louis G. Levoy,

Jr., granted June 17, 1841, and assigned to the assigneeof thisinvention. Reference may be had to this patent for a more completedescription thereof.

As previously stated, capacitor II is connected through unidirectionalcurrent conducting device fig period. During cycles of oppositepolarity, elec tric valve 88 is energized to discharge capacitor it. Toaccomplish this '1?" a, secondary 88 of transformer 88 and. secondary 8?of transformer 88 are connected so as to apply positive voltages to thecontrol element 88 oi electric valve 98 during cycles of a polarityopposite that of thelast half cycle of a welding period. In order torender electric valve 88 conductive, the component of voltage derivedfrom secondary 88 of transformer 88 must be greater than the biasvoltage appearing across resistance element 88 connected in circuittherewith in the control element-cathode circuit of electric valve 88.Fur-- thermore, the component of voltage derived from secondary 8? oftransformer 88 must be of sumcient magnitude to overcome the biasvoltages appearing across resistance elements 88 and 85 also connectedin the control element-cathode circuit of this valve. When both thecomponents of control voltage derived from secondary 88 of transformers88 and secondary 871 of transformer 88 occur at the same time and are ofthe values stated, electric valve 88 will be rendered conductive todischarge capacitor Hi.

If the components of voltage derived from secondary 88 of transformer 88in addition to the bias voltage appearing across resistance element Stis greater than the component of voltage derived fromsecondary 81 oftransformer 88, current will flow through unidirectional currentconducting device I88 producing a voltage drop across resistance elementit" which will equalize the sum of the components of voltages in theparallel connected circuits I8I, 88 and I88, 81, 8|. Under suchcircumstances, the bias voltage across resistance elements 88, 8| ismatched against the voltage derived from secondary 81 of transformer 88and when this latter voltage is greater than this bias voltage, electricvalve 88 will be rendered conductive. If, however, the component ofvoltage derived from secondary 81 of transformer 88 is equal to the sumof the voltages derived from secondary 88 of transformer 88 and fromresistance element 8|, the unidirectional current conducting device I88will become nonconductive and for an interval of time corresponding tothe width of the voltage peak at this time, control will be momentarilytransferred to the circuit including resistance element I8l, secondary-88 of'transformer 88 and resistance element '88 of voltage divider I8,88, 8|.

' That is not until the peak voltage derived from secondary 81 oftransformer 88 renders unidirectional current conducting device I88nonconductive does the control voltage derived from the secondary 88 oftransformer 88 become effective to render electric valve 83 conductive,assuming of course that this voltageof 88 is greater than the biasvoltage across resistance element 88. While unidirectional currentconducting 'device I88 is conductive the control voltage derived fromsecondary 88 of transformer 88 is restrained by the voltage drop acrossresistance element IM and not until this restraint is removed by thevoltage of the secondary 81 of transformer 88 rendering unidirectionalcurrent conducting device I88 nonconductive is control transferred tothe circuit including the secondary 88 of transformer 88. In the aboveconsiderations, the voltage drop across unidirectional currentconducting device I88 has, forconvenience, been considered to be of zerovalue. t

as previously stated, the output voltage vof transformer 88 is of peakedwave form. In phase 13 it is adjusted by means occurs after currentceases to flow in theweldlng circuit as a result or a cycle of voltagecorresponding in polarity to -the last halt cycle 01 a welding period.That is, it must occur after the current flow resulting from this cyclehas decreased to zero and after any hangover flow of current occurringthereafter as a result 01' the inductance of the welding circuit. Inphase it is also adjusted so as to occur concurrently with the flow ofwelding current during a cycle having a polarity opposite thatoi thelast half cycle of energizatlon of the welding circuit. To satisfy ofnetwork I02 so that it this requirement, it must occur late enough insaid half cycle to permit current adjustments to the lowest valuepermissible with the heat control provided for controlling the amount ofcurrent flow in the welding circuit through the agency of electricvalves 1 and 8.

With such adjustments and arrangements it is apparent that even thoughthe component of voltage derived from secondary 96 oi controltransformer 88 varies in magnitude over a wide range, so long as thisvoltage is greater in magnitude than the bias voltage across resistanceelement 80 and so long as the peak voltage wave derived from secondary9'! of control transformer 98 is greater than the bias voltages acrossresistance elements 80 and 8!, electric valve 93 will be renderedconductive. It is of course apparent that the bias voltage acrossresistance element 8i in circuit with the secondary 91 of transformer 98may, under certain circumstances, be omitted. In the arrangementillustrated it has been provided so that the voltage peak derived fromtransformer 98 increases from values below the cathode voltage ofelectric valve 93 to values above such voltage by amounts that aresubstantially equal. Such an arrangement will allow for maximumvariation in the operating characteristics of the electric valvesemployed. I

In view of the above description of the operations of the componentparts of the apparatus shown in Fig. 2, its operation as a whole isbelieved to be apparent. As previously stated, the system functions toimpart a charge to capacitor ll during half cycles of a polaritycorresponding to that of the lasthalf cycle of a welding period and todischarge capacitor H during half cycles of opposite polarity.Consequently, at the end of a welding period capacitor II will retain acharge which, through the agency or electric valve 14, will energize theindicator circuit including milliammeter i5 and provide an indication onmilliammeter i5 informing the welding operator of the value of the lasthalf cycle of voltage across the welding electrodes during the weldingperiod. This indication will be maintained during the deenergizationperiod immediately following a welding period and when the next weldingoperation is initiated, the system will again function to give anindication of the 'value of the last half cycle of voltage of thatwelding period.

It is of course apparent that the signalling system previously describedin connection with Fig. 1 may be embodied in the system of Fig. 2. Inlikemanner, the commutation circuit for electric valve 45 of Fig. 1 maybe associated with the corresponding electric valve 93 of Fig. 2.Various other commutation circuits may be likewise employed. Forexample, sufl'icient inductance may be placed in the anode-cathodecircuits or these valves to insure that they commutate out afterdischarging the capacitors with which they are associated. If suchconductive arrangements are 'employed, the charging rates of thesecapacitors must be such that any reversal of charge thereon due to theinductive circuit, will not revent them from acquiring a charge that isindicative of the voltage across the welding electrodes during halfcycles of voltage corresponding in polarity to that of the last halfcycle of a welding period.

It is also apparent that the loading circuit of Fig. 2 which has beenprovided to prevent direct current saturation of the control transformerhaving its primary connected across the welding electrodes, may likewisebe embodied in the system of Fig. l. I In either system circuits may beprovided for interrupting a welding operation when the voltage across aweld attains a predetermined value. For example, the signalling systemof Fig. 1 may be modified to accomplish this result in accordancev withthe disclosure in United States Letters Patent 2,264,067, Dudley E.Chambers, granted November 25, 1941, and assigned to the assignee ofthis invention.

The above suggested modifications, as well as other modifications, maybe made in the embodiments of my invention illustrated and describedabove without departing from the spirit and scope of my invention. Iaim, consequently, in the appended claims to cover all thosemodifications which come within the true spirit and scope of myinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. Apparatus for use with a load circuit that is intermittentlyenergized from a source of alternating current, said apparatuscomprising means including a device for obtaining a measurement of thelast half cycle value of an electrical condition that occurs in saidload circuit during a period of energization thereof, and means formaintaining the energization of said device and said measurementobtained thereby during the immediately following period ofdeenergization of said load circuit.

2. Apparatus for use with a load circuit that is intermittentlyenergized from a source of alternating current, said apparatuscomprising means including a device for obtaining a measurement of thelast half cycle value of an electrical condition that occurs in saidload circuit during a period of energization thereof, means ifOImaintaining the energization of said device and said measurementobtained thereby during the immediately following period ofdeenergization of said load circuit, and means for indicating when saidmeasurement falls within predetermined limits of magnitude.

3. An alternating current supply circuit, a load circuit, means forintermittently energizing said load circuit from said supply circuit,means responsive only during half cycles of a polarity corresponding tothat of the last half cycle of an energization period of said loadcircuit for obtaining measurements, of the last half cycle values of anelectrical condition that occurs in aseaeee corresponding to that of thelast half cycle of an energization period of said load circuit forobtaining a measurement of the magnitude of the last half cycle value ofan electrical condition in said load circuit, and means for indicatingwhen said measurement falls within predetermined limits of magnitude.

5. Apparatus for use with a load circuit that is intermittentlyenergized from a source of alternating current, said apparatuscomprising a source of voltage, an indicating circuit including anelectric valve having a control element and having its anode-cathodecircuit connected in series with a resistance element across, saidsource of voltage, an indicator connected to be responsive to the amountof current flow through said resistance element in said indicatingcircuit, a capacitor, means for charging said capacitor to a voltageindicative of the magnitude of the last half cycle value of anelectrical condition at said load circuit during its periods ofenergize.- tion, and means for connecting said control element of saidelectric valve to be responsive to the voltage of said capacitor.

6. Apparatus for use with a load circuit that is intermittentlyenergized from a source of alternating current, said apparatuscomprising a source of voltage, an indicating circuit including anelectric valve having a, control element and having its anodecathodecircuit connected in series with a resistance element across said sourceof voltage, an indicator connected to be responsive to the amount oicurrent flow through said resistance element in said indicating cit.-cuit, a capacitor, means for charging said capacitor to a voltageindicative oi the magnitude of the last half cycle value of anelectrical condition of said load circuit during its periods ofenergization, means for connecting the control element of said electricvalve to be responsive to the voltage of said capacitor, and means formaintaining during the period of deenerglzation of said load circuitimmediately following a period of energization thereof thechargeimparted to said capacitor by said last half cycle value of said periodof energizatiom 7. Apparatus for use with a load circuit that isintermittently energized from a source of alternating current, saidapparatus comprising a source of voltage, an indicating circuitincluding an, electric valve having a control element and having itsanode-cathode circuit connected in series with a resistance elementacross' said source of voltage, an indicator connected to be responsiveto the amount of current flow through said resistance element in saidindicating circuit, a capacitor, means responsive to half cycles of apolarity corresponding to the last half cycle of an energization periodof said load circuit for charging said capacitor to a voltage indicativeof the magnitude of the last half cycle value of an electrical conditionof said load circuit during its periods of energization, meansresponsive to half cycles of a polarity opposite that of the last halfcycle of the energization period of said load circuit for dischargingsaid capacitor, and means for connecting the control element ocf saidelectric valve to be responsive to the voltage of said capacitor.

8. Apparatus for producing an indication of the last half cycle value ofan electrical condition of a load circuit which is intermittentlyenergized from a source of alternating current for periods of timeending with half cycles of like polarity, said apparatus comprisingmeans for measuring an electrical value, means responsive to half cyclesof a polarity corresponding to the last half cycle of an energizationperiod of said load circuit for energizing said measuring means inaccordance with the magnitude oi an electrical value that occurs in saidload circuit during said periods of energization, means responsive tohalt cycles of a polarity opposite that of the last half cycle of anenergization period of said load circuit for rendering said lastmentioned means sensitive to electrical values of said load circuitwhich are of smaller magnitude than those previously measured, and meansfor maintaining the tion of said measuring means at the value obtainedfor said last half cycle of an energization. period of said load circuitduring the immediatelyfollowing period of deenergization of said loadcircuit.

9 Apparatus for producing an indication of the last halt cycle value ofan electrical condition of a load circuit which is intermittentlyenergized from a source of alternating current for periods I V of timeending with half cycles of like polarity, said apparatus comprising asource of voltage, an electric valve and an indicating device connectedin series and for energization from said source of voltage, said valveincluding a control member, a capacitor, means for connecting thecontrol member of said electric valve with said capacitor to vary theenergization of said indicating device in accordance with the voltage ofsaid capacitor, means responsive to half cycle val= ues of saidalternating current source having a. polarity corresponding to the lasthalf cycle of an energization period of said load circuit for chargingsaid capacitor to a voltage indicative of the magnitude of the last halfcycle value of an electrical condition of said load circuit during itsperiods of energi'zation, a resistance element and a second electricvalve connected with said capacitor for establishing a discharge circuittherefor, said second electric valve including a control member, andmeans responsive to half cycle val ues of said alternating currentsource having a polarity opposite that of said last half cycle of anenergization period of said load circuit for discharging said capacitor.

10. Apparatus for use with a load circuit that v including an electricvalve having a control element and having its anode-cathode circuitconnected in series with a resistance element across said source ofvoltage, an indicator connected to be responsive to the amount ofcurrent flow through said resistance element in said indicating circuit,a capacitor, means for connecting the control element of said electricvalve to be responsive to the voltage or said capacitor, meansresponsive to half cycles of a polarity corresponding to the last halfcycle of an energization period of said load circuit for charging saidcapacitor to a voltage indicative or the magnitude of the last halfcycle value of an electrical condition 01' said load circuit during itsperiod of energization, a discharge circuit including a second electricvalve having a control element and having its anodecathode circuitconnected in series with a current limiting means across said capacitor,an excitation circuit for said second electric valve connected betweenits control element and its cathode, and means responsive to half cyclesof a polarity opposite that or the last half cycle of the energizationperiod of said load circuit for introducing a control voltage in saidexcitation circuit of said second electric valve which renders saidsecond electric valve conductive and discharges said capacitor.

11. Apparatus for producing an indication of the last half cycle valueof an electrical condition in a translating device which isintermittently energized from a source of alternating current forperiods of time ending with half cycles of like polarity, said circuitcomprising a source of direct current voltage, an electric valve and anindicating device connected in series and for energization from saidsource, said valve including a control member, a capacitor, meansconnecting the control member of said electric valve with said capacitorto vary the current supplied to said indicating device in accordancewith the voltage of said capacitor, means responsive to half cyclevalues of said alternating current source having a polaritycorresponding to the last half cycle of an operating period of saidintermittently energized translating device for charging said capacitorto a voltage indicative of the magnitude of the last half cycle value ofan electrical condition in said translating device during its periods ofenergization, a resistance element and a second electric valve connectedin series with said capacitor for establishing a discharge circuittherefor, said second electric valve including a control member, andmeans responsive to half cycle values of said alternating current sourcehaving a polarity opposite that of said last half cycle of an operatingperiod of said intermittently energized translating device for applyingto the control element of said second electric valve a voltage whichrenders said second electric valve conducting and conditions saidcapacitor for recording the value of an electrical condition in saidtranslating device during half cycles of a polarity corresponding tothat of the last half cycle of an operating period of saidintermittently energized translating device.

12. Apparatus for use with a load circuit that is intermittentlyenergized from a source of alternating current for periods of timeending'with half cycles of like polarity, said apparatus comprising asource of voltage, an indicating circuit including an electric valve ofthe high vacuum type having a control element and having itsanode-cathode circuit connected in series with a resistance elementacross said source of voltage, an indicator connected to be responsiveto the amount of current flow through said resistance element in saidindicating circuit, a capacitor, means for connecting the controlelement of said electric valve to be responsive to the voltage of saidcapacitor, means responsive to half cycles of a polarity correspondingto the last half cycle of an energization period of said load circuitfor charging said capacitor to a voltage indicative of the magnitude ofthe last half cycle value of an electrical condition of said loadcircuit during its periods of energization, a discharge circuitincluding an electric valve of the gaseous discharge type having acontrol element and having its anode-cathode circuit connected in serieswith a resistance element across said capacitor, an excitation circuitconnected between the control element and the cathode of said lastmentioned valve, means responsive to half cycles of a polarity oppositethat of the last half cycle of the energization period of said loadcircuit .for introducing a control voltage in said excitation circuit ofsaid last mentioned electric valve which renders said last mentionedelectric valve conductive and discharges said capacitor, and acommutation circuit for said last mentioned electric valve, saidcommutation circuit being connected across said resistance element inthe anode-cathode circuit of said last-mentioned electric valve andincluding a resistance element and a capacitor connected in series withone another.

13. Apparatus for use with a load circuit that is intermittentlyenergized from a source of alternating current for periods oi timeending with hali cycles 01' like polarity, said apparatus comprising acapacitor, means connected to said capacitor and responsive to haltcycles of voltage across said load circuit corresponding in polarity tothe last half cycle of its periodic energization for charging saidcapacitor, a discharge circuit for said capacitor, said dischargecircuit including an electric valve having a control element and havingits anode-cathode circuit connected in series with a current limitingmeans across said capacitor, means including a saturable inductivedevice energized in response to the flow of current through said loadcircuit for producing a voltage of peaked wave form on the controlelement oi said electric valve when the current in said load circuitpasses through zero value, said inductive device being connected so thatsaid electric valve is rendered conductive at the end of a half cycle ofcurrent flow resulting from half cycles of voltage or a polarityopposite that of the last half cycle of an energization period of saidload circuit, a source of voltage, an indicating circuit including asecond electric valve having a control element and having itsanodecathode circuit connected in series with a resistance elementacross said source of voltage, an indicator connected to be responsiveto the amount of current flow through said resistance element in saidindicating circuit, and means for connecting the control element of saidsecond electric valve to be responsive to the voltage of said capacitor.

14. Apparatus for use with a load circuit that is intermittentlyenergized from a source of alternating current for periods of timeending with half cycles of like polarity, said apparatus comprising acapacitor, means responsive to half cycles of voltage across said loadcircuit corresponding in polarity to the last half cycle of its periodicenergization for charging said capacitor, means including a time delayrelay for completing the connection of said capacitor with said lastmentioned means upon the energization of said load circuit and formaintaining said connection for a predetermined interval of time aftersaid load circuit is deenergized, a discharge circuit for saidcapacitor, said discharge circuit including an electric valve having acontrol ele ment and having its anode-cathode circuit connected inseries with a current limiting means across said capacitor, meansincluding a saturable inductive device energized in response to the flow01' current through said load circuit for producing a voltage of peakedwave form on tne control.element of said electric valve when the currentin said load circuit passes through zero value, said inductive devicebeing connected so that said electric valve is rendered conductive atthe end of a half cycle of current flow resulting from half cycles ofvoltage of a polarity opposite that of the last half cycle of anenergization .period or said load circuit, a source or voltage, anindicating circuit including a second electric valve having a controlelement and havis intermittently energized from a source of alternatingcurrent for periods oi time ending with hali cycles of like polarity,said apparatus come prising a capacitor, means responsive to half cyclesof a polarity corresponding to the last half cycle of the energizationperiod of said load circuit for charging said capacitor to a voltageindicative of the magnitude of said halt cycle value of an electricalcondition of said load circuit, time delay means for completing theconnection of said capacitor with said last mentioned means upon theenergization of said load circuit and for maintaining said connectionfor a predetermined interval oi time after said load circuit isdeenergized, a' discharge circuit for said capacitor',said dischargecircuit including an electric valve having a control element and havingits anode-cathode circuit connected in series with a current limitingand commutating means across said capacitor, means responsive to halfcycles of a polarity opposite that of the last half cycle of anenergization period of said load circuit 101' applying a voltage to saidcontrol element of said electric valve which renders it conductive, asource of voltage, an indicating circuit including a second electricvalve having a control element and having its anode-cathode circuitconnected in series with a resistance element across said source ofvoltage, an indicator connected to be responsive to the amount ofcurrent flow through said resistance element in said indicating circuit,and means for connecting said control element of said second electricvalve to be responsive to the voltage of said capacitor.

16. Apparatus comprising a source of alternating current, a loadcircuit, means for periodically energizing said load circuit from saidsource of alternating current. a. capacitor, a circuit .ior chargingsaid capacitor comprising a unidirectional current conducting deviceconnected across the secondary winding of a transformer having itsprimary winding connected to be responsive to an electrical condition insaid load circuit, said unidirectional current conducting device beingpoled to charge said condenser in response to half cycles of voltageacross said load circuit corresponding in polarity to the last halfcycle of its periodic energization, current limiting means and a secondunidirectional current conducting device connected in series with oneanother across the secondary winding of said transformer. said secondunidirectional current conducting device being poled to conduct currentthrough said current limiting means in response to half cycles ofvoltage across said load circuit of a polarity opposite to that of thelast half cycle of its periodic'energization, a discharge circuit forsaid capacitor, said discharge circuit including an electric valvehaving a control element and having its anode-cathode circuit connectedin series with a current limiting means across said capacitor, meansresponsive to half cycles of a polarity opposite that of the last halfcycle of the ener'gization periods of said load circult for applying acontrol voltage to said control element oi said electric valve andrenderaccuses Bil lid

ing it conductive, a'source of voltage, an indicating circuit includinga second electric valve having a control element and having itsanodecathode circuit connected in series with a resistance elementacross said source of voltage, an indicator connected to be responsiveto the amount of current flow through said resistance element in saidindicating circuit, and means for connecting said control element ofsaid second electric valve to be responsive to the voltage of saidcapacitor.

17. Apparatus comprising an indicator, means including a source ofvoltage for controlling the energization of said indicator, an electricvalve having a control element and having its anodecathode circuitenergized from said source of voltage, and two parallel connectedexcitation circuits for said electric valve only one of which ispolarized and includes a unidirectional cur rent conducting devicehaving one terminal connected to said control element oi said electricvalve and its other terminal connected in series circuit with a sourceof control voltage to the cathode of said electric valve and the otherof which includes a resistance element connected in series circuit witha second source of control voltage, said unidirectional currentconductin device of said polarized excitation circuit bein poled toconduct current from said non-polarized excitation circuit through saidresistance element thereof.

18.'Apparatus for producing an indication of the magnitude of anelectrical condition in a load circuit that is periodically energizedfrom a supply circuit, said apparatus comprising a capacitor, means forcharging said capacitor to a voltage indicative of the magnitude of anelec trical condition in said load circuit, means responsive to thevoltage of said capacitor for indicating the magnitude of saidelectrical condition in said load circuit, means for discharging.

said capacitor including an electric valve having a control element andhaving its anodecathode circuit connected across said capacitor, anexcitation circuit for said electric valve including a resistanceelement having one terminal connected to the control element of saidelectric valve and its other terminal connected in circuit with a sourceof control voltage and a source of direct current bias voltage whosepositive terminal is connected to the cathode of said electric valve,and a second excitation circuit for said electric valve including a.unidirectional current conducting device having one terminal connectedto said control element of said electric valve and its other terminalconnected in circuit with a second source of periodic control voltageand a second source of direct current bias voltage having its positiveterminal connected to the negative terminal of said first mentionedsource of bias voltage, said unidirectional current conducting devicebeing poled to conduct current from the terminal of said resistanceelement connected to the control element of said electric valve, saidfirst source of control voltage being derived from said load circuit.and said second source of control voltage be ing derived from saidsupply circuit and having a peaked wave form whose maximum value isgreater than the sum of the voltages of said sources of bias voltage.

19. Apparatus comprising a capacitor, means for charging said capacitorin response to an electrical condition of a periodically energized loadcircuit, means for discharging said capacacross said capacitor, anexcitation circuit for said electric valve including a resistanceelement having one terminal connected to the control element of saidelectric valve and its other terminal connected in circuit with a sourceof control voltage and a source of direct current bias voltage whosepositive terminal is connected to the oathode of said electric valve, asecond excitation circuit for said electric valve including aunidirectional current conducting device, having one terminal connectedto said control element of said electric valve and its other terminalconnected in circuit with a second source of periodic control voltageand a second source of direct current bias voltage having its positiveterminal connected to the negative terminal of said first mentionedsource or bias voltage, said unidirectiona1 current conducting devicebeing poled to conduct current from the terminal of said resistanceelement connected to the control element of said electric valve, saidfirst mentioned source of control voltage being derived from said loadcircuit, and said second source of control voltage having a peaked waveform whose maximum value is greater than the sum of the'voltages of saidsources of bias voltage, a source of voltage, an indicating circuitincluding a second electric valve having a control element and havingits anode-cathode circuit connected in series with a resistance elementacross said source of voltage, an indicator connected to be responsiveto the amount of current flow through said resistance element in saidindicating circuit, and means for connecting the control element oi saidsecond electric valve to be responsive to the voltage .of saidcapacitor.

20. Apparatus comprising a source of altemating current, a load circuit,means for periodically energizing said load circuit from said source ofalternating current, atransiormer having a plurality of secondarywindings and a primary winding connected to be energized in response toa voltage condition of said load circuit, a capacitor, a circuit forcharging said capacitor comprising a unidirectional current conductingdevice connected in series with said capacitor across one of thesecondary windings or said transformer, said unidirectional currentconducting device poled to charge said condenser in response to haltcycles of voltage corresponding in polarity to the last half cycle ofthe energization periods of said load circuit, a loading circuitincluding a second unidirectional current conducting device connectedacross said one of said secondary windings 01 said transformer, saidsecond unidirectional current conducting device being poled to conductin response to halt cycles of voltage of a polarity opposite thepolarity of the last half cycle or the energization periods of said loadcircuit, a circuit for discharging said capacitor including an electricvalve having a control element and having its anode-cathode circuitconnected in series with a current limiting means across said capacitor,a resistance elementhaving one terminal connected to the control elementof said electric valve and its other terminal connected in circuit withanother secondary winding of said transformer and a source of directcurrent bias voltage the positive terminal of which is connected to thecathode of said electric valve, a second transformer of the saturabletype having a secondary winding and a primary winding connected througha phase shift circuit to said source of alternating current, a thirdunidirectional current conducting device having one terminal connectedto said control element of said electric valve and its other terminalconnected in circuit with the secondary winding of said secondtransformer and a second source of direct current bias voltage thepositive terminal of which is connected to the negative terminal of saidfirst mentioned source of bias voltage, said third unidirectionalcurrent conducting device being poled to conduct current from theterminal of said resistance element which is connected to said controlelement of said electric valve and the peaked voltage of the secondarywinding of said second control transformer having a polaritycorresponding to that of said other secondary winding of said firstmentioned control transformer and being greater in value than the sum ofthe-voltages of said sources of bias voltage, a source of voltage, anindicating circuit including a second electric valve having a controlelement and having its anode-cathode circuit connected in series with asecond resistance element across said source of voltage, an indicatorconnected to be responsive to the amount or current flow through saidsecond resistance element in said indicating circuit,

, and means for connecting the control element or said second electricvalve to be responsive to the voltage of said capacitor.

. ORRIN W. LIVINGSTON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,014,102 Conklin Sept. 10, 19352,143,219 Wenger Jan. 10, 1939 2,147,918 Overbeck Feb. 21, 19392,190,743 Vance Feb. 20, 1940 2,264,067 Chamber Nov. 25, 1941 2,269,151Garman Jan. 6, 1942 2,287,926 Zepler June 30, 1942 2,300,198 Brown Oct.27, 1942 2,309,560 Welty Jan. 26, 1943 2,320,916 Dawson June 1, 19432,325,390 Garman July 2'7, 1943 2,331,229 Regan Oct. 5, 1943 OTHERRams-canons Vacuum Tube Voltmeters, (1941) by John I". Rider, (page 58),published by John F. Rider Publisher, Inc., New York, N. Y.

Welding Journal," vol. 19 (1940), pages 215-218.

